1. Field of the Invention
This invention relates to an improvement in electromagnetic railgun construction which substantially improves railgun performance and permits use of a solid armature. In particular, it relates to a railgun having a layer of low conductivity material, such as graphite, along the railgun bore which inhibits the concentration of current density along the interface of the rail and armature.
2. Description of the Related Art
Thermodynamic guns are widely used and generally understood in a broad context. In an ordinary thermodynamic gun, a propellant burns to generate high pressure gas that pushes a projectile down a bore. While thermodynamic guns are used in many applications besides weapons--for example scientific and industrial applications--their use is somewhat limited because of the maximum velocities attainable. Thus, physical limitations limit the projectile from such thermodynamic guns from reaching velocities much greater than one kilometer per second.
Electromagnetic railguns have been widely investigated since World War II as an alternative to thermodynamic guns because of the possibilities of achieving projectile hypervelocities (greater than one kilometer per second).
The early electromagnetic railguns incorporated a solid armature which was propelled between the rails by the electromagnetic force generated by the current flow through the armature and the rails. However, it was soon found that at high speeds around one kilometer per second, the rails and armature were substantially damaged, possibly as a result of ohmic heating and/or internal forces. Further, increases in current flow tended to only increase rail and armature gouging without an increase in armature velocity. Thus, armature velocities in excess of one kilometer per second were not practically attainable for railguns using solid armatures.
In the early 1970's, R. A. Marshall, J. P. Barber, and others at the Australian National University, Canberra, Australia, developed railguns using plasma armatures which could obtain hypervelocities and could make efficient use of high current, pulsed power supplies, such as compulsators and homopolar generators. Plasma armature railguns are generally described in S. C. Rashleigh and R. A. Marshall, "Electromagnetic Acceleration of Macroparticles to High Velocities," 49 J. App. Phys. 2540 (Apr. 1978) (incorporated herein by reference). Such power supplies are broadly illustrated in U.S. Pat. Nos. 4,200,831; 4,459,504; 4,246,507; and U.S. patent application Ser. No. 06/689,868 issued U.S. Pat. No. 4.026,286 (incorporated herein by reference).
In recent years, additional research has revealed numerous problems associated with very high current plasma armatures. For example, at the high currents necessary to obtain hypervelocities, rail errosion has been a significant problem which essentially relegates the railgun to a one or two shot application. Additionally, plasma armature type railguns require a sealed bore (open only at the muzzle) capable of withstanding the substantial electromagnetic forces generated; the gaskets, seals, and insulator material associated with such bores have proven a significant problem. For example, in addition to rail and insulator damage, metallic deposits often adhere to the insulator surfaces after firing, causing arcing problems during subsequent firing.
In addition to the practical difficulties of plasma-type electromagnetic railguns, several fundamental problems have become of increasing concern. For example, at the typical working currents in question, the dissipative armature voltage drop is an order of magnitude greater than desirable. Perhaps more fundamentally, plasma armatures are designed to accelerate a projectile using base pressures. Base pressure acceleration (such as also used in thermodynamic guns) places severe design limits on the projectile. For example, a projectile must be able to withstand the extreme temperature and pressures exerted at its base by the driving plasma. Such projectile design limits are removed and gun efficiency substantially improved if a solid armature is used as the projectile. A solid armature/projectile offers the prospect of acceleration by body forces on the armature/projectile.
While a solid armature railgun avoids many of the problems associated with plasma armatures--e.g. substantial dissipative armature voltage drop and base pressure acceleration--attempts to obtain hypervelocities with solid armature railguns have proven unsuccessful. Thus, it would a significant advance in the art if a railgun were devised which could utilize a solid armature and operate at velocities substantially in excess of one kilometer per second.